Structure of porous low-k layer and interconnect structure

ABSTRACT

A structure of a porous low-k layer is described, comprising a bottom portion and a body portion of the same atomic composition, wherein the body portion is located on the bottom portion, and the bottom portion has a density higher than the density of the body portion. An interconnect structure is also described, including the above porous low-k layer, and a conductive layer filling up a damascene opening in the porous low-k layer.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional application of and claims the prioritybenefit of U.S. application Ser. No. 11/672,307, filed on Feb. 7, 2007,now allowed. The entirety of the above-mentioned patent application ishereby incorporated by reference herein and made a part of thisspecification.

BACKGROUND OF THE INVENTION

1. Field of Invention

This invention relates to an integrated circuit (IC) fabricating processand related structures, and more particularly to a forming method and astructure of a porous low-k layer, an interconnect process and aninterconnect structure.

2. Description of Related Art

As the linewidth of IC devices is unceasingly reduced, the affect to theRC delay effect to the speed of the devices continuously becomes larger.One way to reduce the RC delay effect is to decrease the parasitecapacitance in the interconnect structure, and the parasite capacitancemay be decreased by decreasing the dielectric constant of the dielectriclayers in the interconnect structure, i.e., by forming the dielectriclayers from a low-k material that has a dielectric constant lower thanthat (≈4.0) of silicon oxide.

Currently, the low-k materials frequently used include organic low-kmaterials, porous low-k materials and so on, wherein a porous low-kmaterial may be formed with a sol-gel method, a spin-on method or achemical vapor deposition (CVD) method that usually uses a frameworkprecursor for forming the framework of the porous structure and aporogen (or a porogen precursor). The porogen will be removed after theporous low-k layer is formed.

Though the dielectric constant of a porous low-k layer can be lowerbelow 2.0, a porous low-k layer easily causes a undesired etchingprofile possibly because of its low density as compared with non-porousmaterials and the resulting etching rate difference between the porouslow-k layer and the adjacent films. For example, a porous low-k layermay cause an undesired etching profile in an etching step for forming adamascene opening in a damascene process, as shown in FIG. 1.

Referring to FIG. 1, in the damascene process, a porous low-k layer 120and a hard mask layer 130 are sequentially formed on a substrate 100having thereon a conductive layer 110 to be connected. A via hole 140 isthen formed in the hard mask layer 130 and the porous low-k layer 120through anisotropic etching, and then the via hole 140 is filled with aconductive material to form a conductive plug (not shown). Possiblybecause the etching rate difference between the porous low-k layer 120and the non-porous hard mask layer 130 is large, a kink profile 132easily occurs to the hard mask layer 130 around the via hole 140. Thekink profile 132 will interfere with the filling of the conductivematerial later, so that the quality of the resulting is lowered.

Moreover, when a cap layer is disposed under the porous low-k layer andthe damascene opening has to be formed through the cap layer, a kinketching profile also occurs to the cap layer. Referring to FIG. 4, adamascene opening 440 exposing a portion of the conductive layer 410 tobe connected is formed through a hard mask layer 430, a porous low-klayer 420 and a cap layer 415 on the substrate, wherein the cap layer415 has a kink profile 417 and the hard mask layer 430 has a kinkprofile 432. The two kink profile 417 and 432 both interfere with thefilling of the conductive material.

SUMMARY OF THE INVENTION

Accordingly, this invention provides a method of forming a porous low-klayer that does not cause an undesired etching profile, especially akink profile.

This invention also provides an interconnect process that can utilizethe method of forming a porous low-k layer of this invention to form theporous low-k layer therein.

This invention further provides a structure of a porous low-k layer,which does not cause an undesired etching profile and can be formed withthe method of forming a porous low-k layer of this invention.

This invention further provides an interconnect structure that can beformed with the interconnect process of this invention.

The method of forming a porous low-k layer of this invention isdescribed below. A CVD process in which a framework precursor and aporogen precursor are supplied is performed to a substrate. In an endperiod of the supply of the framework precursor, the value of at leastone deposition parameter negatively correlated with the density of aproduct of the CVD process is decreased.

It is preferred that the porogen size in the CVD process does not exceed100 Å. It is noted that in this invention, the size of a porogen isdefined by the size of the pore in the porous low-k layer that is causedby the porogen.

In some embodiment, the deposition parameter is the flow rate of theporogen precursor. In the end period, the flow rate of the porogenprecursor may be set to zero or a fixed positive value, or may bedecreased to 0 or a positive value with time in two or more steps. It ispreferred that the CVD process deposits a thickness no more than 250 Åin any period in which the flow rate of the porogen precursor is zero.

In some embodiments, the deposition parameter is the porogen size. Theporogen size is constant, or is decreased with time in two or moresteps, in the end period.

In certain cases, the above method of forming a porous low-k layer ofthis invention may further include setting the value of the depositionparameter in an initial period of the supply of the framework precursorthat is smaller than the value set after the initial period but beforethe end period. When the deposition parameter is the flow rate of theporogen precursor. The flow rate of the porogen precursor may be set tozero or a fixed positive value, or may be increased from 0 or a positivevalue with time in two or more steps, in the initial period. When thedeposition parameter is the porogen size, the porogen size may beconstant, or may be increased with time in two or more steps, in theinitial period.

The interconnect process of this invention is described below. Asubstrate having thereon a conductive layer to be connected is provided.A CVD process is utilized to form a porous low-k layer on the substrate,wherein the porous low-k layer includes a top portion, a bottom portionand a body portion of the same atomic composition, the body portion isbetween the top portion and the bottom portion, and the top portion hasa density higher than that of the body portion. A hard mask layer isformed on the porous low-k layer. A damascene opening is formed in thehard mask layer and the porous low-k layer exposing at least a portionof the conductive layer to be connected. A conductive material layer isformed over the substrate filling up the damascene opening.

In some embodiments of the above interconnect process, a frameworkprecursor and a porogen precursor are supplied in the CVD process. In anend period of the supply of the framework precursor, the value of atleast one deposition parameter negatively correlated with the density ofa product of the CVD process is decreased. In addition, when thesubstrate is provided with a cap layer thereon covering the conductivelayer to be connected, the bottom portion also has a density higher thanthat of the body portion. To make the bottom portion have a densityhigher than that of the body portion, the value of the depositionparameter is set, in an initial period of the supply of the frameworkprecursor, smaller than the value set after the initial period butbefore the end period.

The structure of a porous low-k layer of this invention includes a topportion, a bottom portion and a body portion of the same atomiccomposition. The body portion is between the top portion and the bottomportion. The top portion has a density higher than that of the bodyportion.

The interconnect structure of this invention includes a substrate havinga first conductive layer thereon, an above-mentioned porous low-k layerand a second conductive layer. The top portion, the bottom portion andthe body portion together have a damascene opening therein over thefirst conductive layer. The second conductive layer fills up thedamascene opening and contacts with the first conductive layer.

The pore size of the porous low-k layer preferably does not exceed 100Å. In an embodiment, the top portion has no pore therein. The thicknessof such a top portion preferably does not exceed 250 Å. In anotherembodiment, the pore number density in the top portion of the porouslow-k layer increases from zero or a positive value in two or more stepsin a depth direction of the porous low-k layer, and the pore size in thetop portion is equal to that in the body portion. The thickness of afraction of such a top portion where the pore number density is zero ispreferably no more than 250 Å. It is noted that in this invention, apore number density is defined as the number of pores in unit volume ofthe CVD product. When the pore size is fixed, a higher pore numberdensity means a lower density (mass/volume) for a porous material.

In still another embodiment, the pores in the top portion have one firstsize, the pores in the body portion has one second size, and the onefirst size is smaller than the one-second size. In still anotherembodiment, the pore size in the top portion increases from a positivevalue in two or more steps in a depth direction of the porous low-klayer, but is smaller than a pore size in the body portion.

The interconnect structure of this invention may further include a caplayer located between the substrate and the porous low-k layer andpenetrated by the second conductive layer, wherein the bottom portionalso has a density higher than that of the body portion. In anembodiment, the bottom portion has no pore therein. In anotherembodiment, the pore number density in the bottom portion decreases tozero or, a positive value in two or more steps in the depth direction ofthe porous low-k layer, and the pore size in the bottom portion is equalto that in the body portion. In still another embodiment, the pores inthe bottom portion has one first size, the pores in the body portion hasone second size, and the one first size is smaller than the one secondsize. In still another embodiment, the pore size in the bottom portiondecreases to a positive value in two or more steps in the depthdirection of the porous low-k layer and is smaller than a pore size inthe body portion.

Since the top portion of the porous low-k layer directly connected withthe hard mask layer has a density higher than that of the body portionin the interconnect process, in the etching process for forming thedamascene opening, the top portion can have an etching rate closer tothat of the hard mask layer so that a kink profile does not easily occurto the hard mask layer and the opening can be easily filled with aconductive material.

Moreover, when the substrate is provided further having a cap layerthereon covering the conductive layer to be connected, the bottomportion of the porous low-k layer directly connected with the cap layermay also have a density higher than that of the body layer. Therefore,in the etching process for forming the damascene opening, the topportion and the bottom portion each can have an etching rate closer tothat of the hard mask layer so that a kink profile does not easily occurto the hard mask layer or the cap layer and the opening can be easilyfilled with a conductive material.

In order to make the aforementioned and other objects, features andadvantages of the present invention comprehensible, a preferredembodiment accompanied with figures is described in detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a kink profile formed in an etching step which forms adamascene opening in a stack of a porous low-k layer and a hard masklayer thereon in the prior art.

FIG. 2 depicts an interconnect process and an interconnect structureaccording to a first embodiment of this invention.

FIG. 3 shows various time-dependent profiles for the parameter that isnegatively correlated with the density of the CVD product according tothe first embodiment of this invention.

FIG. 4 depicts a kink profile formed in an etching step which forms adamascene opening in a stack of a cap layer, a porous low-k layer and ahard mask layer in the prior art.

FIG. 5 depicts an interconnect process and an interconnect structureaccording to a second embodiment of this invention.

FIG. 6 shows various time-dependent profiles for the parameter that isnegatively correlated with the density of the CVD product in the initialperiod of the CVD process according to the second embodiment of thisinvention.

DESCRIPTION OF EMBODIMENTS First Embodiment

FIG. 2 depicts an interconnect process and an interconnect structureaccording to the first embodiment of this invention. In the interconnectprocess, a substrate 200 is provided with a conductive layer 210 to beconnected thereon, wherein the material of the conductive layer 210 maybe copper. A CVD process is performed to form a porous low-k layer 220on the substrate 200 covering the conductive layer 210 and including atop portion 220 a, a body portion 220 b and a bottom portion 220 c ofthe same atomic composition (e.g., SiO₂). The body portion 220 b isbetween the top portion 220 a and the bottom portion 220 c, and the topportion 220 a has a density higher than that of the body portion 220 b.

Referring to FIG. 2 again, a hard mask layer 230 is formed on the porouslow-k layer 220, possibly including silicon nitride formed throughPECVD. A via hole 240 is then formed through the hard mask layer 230 andthe porous low-k layer 220, at least exposing a portion of theconductive layer 210 to be connected. A barrier layer 250 and aconductive material layer 260 is then formed over the substrate 200. Thebarrier layer 250 is conformal with the substrate surface, possiblyincluding Ti/TiN and possibly formed with MOCVD. The conductive materiallayer 260 fills up the via hole 240, possibly including copper andpossibly formed through electroplating.

The subsequent steps include removing a portion of the conductivematerial layer 260 and a portion of the barrier layer 250 higher thanthe top of the porous low-k layer 220 and the whole hard mask layer 230to form a conductive plug 260 a, possibly with a chemical mechanicalpolishing (CMP) process.

When the above CVD process utilizes a framework precursor and a porogenprecursor to form the porous low-k layer 220, the porogen in the poreshas to be removed, possibly through heating, UV irradiation or e-beamirradiation, after the CVD process to reduce the dielectric constant ofthe porous low-k layer 220. The method of making the density of the topportion 220 a of the porous low-k layer 220 higher than that of the bodyportion 220 b may include decreasing the value of at least onedeposition parameter negatively correlated with the density of the CVDproduct in an end period of the supply of the framework precursor. Thetop portion 220 a is the portion of the porous low-k layer 220 that isdeposited in the end period.

When the deposition parameter is the flow rate of the porogen precursor,in the above end period, the flow rate may be set to zero, as shown inFIG. 3( a), or may be set to a fixed positive value, or may be decreasedto zero or a positive value with time in two or more steps, as shown inFIG. 3( b) or FIG. 3( c).

When the flow rate of the porogen precursor is set to zero in the endperiod, the top portion 220 a of the porous low-k layer 220 has no poretherein. The thickness of such a top portion 220 a is preferably no morethan 250 Å so that the porogen in the underlying body portion 220 b andthe bottom portion 220 c can be removed effectively. On the other hand,the lower limit of the thickness of such a top portion 220 a may be 200Å.

When the flow rate of the porogen precursor is decreased to zero or apositive value with time in two or more steps, the pore size in the topportion 220 a is the same as that in the body portion 220 b. Since aportion of the porous low-k layer 220 at a larger depth is depositedearlier and more porogen precursor means more porogen produced in theCVD process and more pores in the CVD product, the pore number densityin the top portion 220 a increases from 0 or a positive value in two ormore steps in the depth direction of the porous low-k layer 220.

When the above deposition parameter is the porogen size, in the endperiod, the porogen size can be constant or be decreased in two or moresteps with time, as shown in FIG. 3( d) and FIG. 3( e). When the porogensize is constant in the end period, the one pore size of the top portion220 a is smaller than the one pore size of the body portion 220 b. Whenthe porogen size is decreased in two or more steps with time, the poresize in the top portion 220 a increases from a positive value but issmaller than th pore size in the body portion 220 b.

The porogen size is varied usually by changing the species of theintroduced porogen precursor to obtain a porogen of a predeterminedsize. An exemplary set of porogens with different sizes is shown below.

The porogens A, B and C cause pore sizes of 13 Å, 14 Å and 15 Å,respectively, and are therefore considered as porogens with differentsize. The porogen size is usually no more than 100 Å, preferably no morethan 30 Å.

Moreover, it is also possible in this invention to decrease two or moredeposition parameters negatively correlated with the density of the CVDproduct in the end period. For example, it is possible to decrease thepore size to a constant value and then set the flow rate of the porogenprecursor to zero, as shown in FIG. 3( f), or to decrease the pore sizewith time in two or more steps and then set the flow rate of the porogenprecursor to zero, as shown in FIG. 3( g).

Second Embodiment

FIG. 5 depicts an interconnect process and an interconnect structureaccording to the second embodiment of this invention. In theinterconnect process, a substrate 500 is provided with a conductivelayer 510 to be connected thereon, wherein the material of theconductive layer 510 may be copper. A cap layer 515 is then formed onthe substrate 500 covering the conductive layer 510 to be connected,possibly including silicon nitride and possibly formed with PECVD. A CVDprocess is performed to form a porous low-k layer 520 on the cap layer515, including a top portion 520 a, a body portion 520 b and a bottomportion 520 c of the same atomic composition (e.g., SiO₂). The bodyportion 520 b is between the top portion 520 a and the bottom portion520 c, and each of the top portion 520 a and the bottom portion 520 chas a density higher than that of the body portion 520 b.

Referring to FIG. 5 again, a hard mask layer 530 is formed on the porouslow-k layer 520, possibly including silicon nitride formed throughPECVD. A via hole 540 is then formed through the hard mask layer 530,the porous low-k layer 520 and the cap layer 515, at least exposing aportion of the conductive layer 510 to be connected. A barrier layer 550and a conductive material layer 560 are sequentially formed over thesubstrate 500, wherein the shape, materials and forming methods of thebarrier layer 550 and the conductive material layer 560 may be the sameas those in the first embodiment. The subsequent steps include removinga portion of the conductive material layer 560 and a portion of thebarrier layer 550 higher than the top of the porous low-k layer 520 andthe whole hard mask layer 530 to form a conductive plug 560 a, possiblywith a chemical mechanical polishing (CMP) process.

When the above CVD process utilizes a framework precursor and a porogenprecursor to form the porous low-k layer 520, the porogen in the poreshas to be removed, possibly through heating, UV irradiation or e-beamirradiation, after the CVD process to reduce the dielectric constant ofthe porous low-k layer 520. The method of making the density of the topportion 220 a of the porous low-k layer 220 higher than that of the bodyportion 220 b may be the same as that shown in the first embodiment andFIG. 3, i.e., decreasing the value of at least one deposition parameternegatively correlated with the density of the CVD product in an endperiod of the supply of the framework precursor.

On the other hand, the method of making the density of the bottomportion 520 c of the porous low-k layer 520 higher than that of the bodyportion 520 b may include setting, in an initial period of the supply ofthe framework precursor, the value of at least one deposition parameternegatively correlated with the density of the CVD product to be smallerthat that set after the initial period but before the end period. Thebottom portion 520 c is the portion of the porous low-k layer 520 thatis deposited in the initial.

When the deposition parameter is the flow rate of the porogen precursor,in the above initial period, the flow rate may be set to zero, as shownin FIG. 6( a), or may be set to a fixed positive value, or may beincreased from zero or a positive value with time in two or more steps,as shown in FIG. 6( b) or FIG. 6( c).

When the flow rate of the porogen precursor is set to zero in theinitial period, the bottom portion 520 c of the porous low-k layer 520has no pore therein. When the flow rate of the porogen precursor isincreased from zero or a positive value with time in two or more steps,the pore size in the bottom portion 520 c is the same as that in thebody portion 520 b. Since a portion of the porous low-k layer 520 at alarger depth is deposited earlier and more porogen precursor means moreporogen produced in the CVD process and more pores in the CVD product,the pore number density in the bottom portion 520 c decreases to 0 or apositive value in two or more steps in the depth direction of the porouslow-k layer 520.

When the above deposition parameter is the porogen size, in the initialperiod, the porogen size can be constant or be increased with time intwo or more steps, as shown in FIG. 6( d) and FIG. 6( e). When theporogen size is constant in the initial period, the one pore size of thebottom portion 520 c is smaller than the one pore size of the bodyportion 520 b. When the porogen size is increased with time in two ormore steps, the pore size in the bottom portion 520 c decreases from apositive value and is smaller than the pore size in the body portion 520b. The porogen size may be varied with the same method mentioned in thefirst embodiment. The porogen size is usually no more than 100 Å,preferably no more than 30 Å.

Moreover, it is also possible in this embodiment to decrease two or moredeposition parameters negatively correlated with the density of the CVDproduct in the end period. For example, it is possible to set the flowrate of the porogen precursor to zero and then set the pore size to aconstant value, as shown in FIG. 6( f), or to set the flow rate of theporogen precursor to zero and then increase the pore size with time intwo or more steps, as shown in FIG. 6( g). In the above two cases, thelower-half fraction of the bottom portion 520 c has no pore therein.

In summary, in the above first and second embodiments, the top portionof the porous low-k layer directly connected with the hard mask layerhas a density higher than that of the body portion in the interconnectprocess. Hence, in the etching process for forming the damasceneopening, the top portion can have an etching rate closer to that of thehard mask layer so that a kink profile does not easily occur to the hardmask layer and the opening can be easily filled with a conductivematerial.

Moreover, when the substrate is provided further having a cap layerthereon covering the conductive layer to be connected as in the secondembodiment, the bottom portion of the porous low-k layer directlyconnected with the cap layer may also have a density higher than that ofthe body layer. Hence, in the etching process for forming the damasceneopening, the top portion and the bottom portion each can have an etchingrate closer to that of the adjacent layer so that a kink profile doesnot easily occur to the hard mask layer or the cap layer and the openingcan be easily filled with a conductive material.

The present invention has been disclosed above in the preferredembodiments, but is not limited to those. It is known to persons skilledin the art that some modifications and innovations may be made withoutdeparting from the spirit and scope of the present invention. Therefore,the scope of the present invention should be defined by the followingclaims.

1. A structure of a porous low-k layer, comprising a porous bottomportion and a body portion of the same atomic composition, wherein thebody portion is located on the porous bottom portion, and the porousbottom portion has a density higher than a density of the body portion.2. The structure of claim 1, wherein a pore size in the porous low-klayer does not exceed 100 Å.
 3. The structure of claim 1, wherein in theporous bottom portion of the porous low-k layer, the pore number densitydecreases to zero or a positive value in two or more steps in a depthdirection of the porous low-k layer, and a pore size in the porousbottom portion is equal to that in the body portion.
 4. The structure ofclaim 1, wherein the pores in the porous bottom portion has one firstsize, the pores in the body portion has one second size, and the onefirst size is smaller than the one second size.
 5. The structure ofclaim 1, wherein in the porous bottom portion, the pore size decreasesto a positive value in two or more steps in a depth direction of theporous low-k layer and is smaller than a pore size in the body portion.6. The structure of claim 1, further comprising a top portion of thesame atomic composition that is located on the body portion and has adensity higher than the density of the body portion.
 7. The structure ofclaim 6, wherein the top portion has no pore therein.
 8. The structureof claim 7, wherein a thickness of the top portion does not exceed 250Å.
 9. The structure of claim 6, wherein in the top portion of the porouslow-k layer, the pore number density increases from zero or a positivevalue in two or more steps in a depth direction of the porous low-klayer, and a pore size in the top portion is equal to that in the bodyportion.
 10. The structure of claim 9, wherein a fraction of the topportion where the pore number density is zero has a thickness no morethan 250 Å.
 11. The structure of claim 6, wherein the pores in the topportion has one first size, the pores in the body portion has one secondsize, and the one first size is smaller than the one second size. 12.The structure of claim 6, wherein in the top portion, the pore sizeincreases from a positive value in two or more steps in a depthdirection of the porous low-k layer but is smaller than a pore size inthe body portion.
 13. An interconnect structure, comprising: asubstrate, having a first conductive layer thereon; a porous low-k layerthat comprises a porous bottom portion and a body portion of the sameatomic composition, wherein the body portion is located on the porousbottom portion, the porous bottom portion has a density higher than adensity of the body portion, and the porous bottom portion and the bodyportion together have a damascene opening therein over the firstconductive layer; and a second conductive layer filling up the damasceneopening and contacting with the first conductive layer.
 14. Theinterconnect structure of claim 13, wherein a pore size in the porouslow-k layer does not exceed 100 Å.
 15. The interconnect structure ofclaim 13, further comprising a cap layer located between the substrateand the porous low-k layer and penetrated by the second conductivelayer.
 16. The interconnect structure of claim 13, wherein in the porousbottom portion of the porous low-k layer, the pore number densitydecreases to zero or a positive value in two or more steps in a depthdirection of the porous low-k layer, and a pore size in the porousbottom portion is equal to that in the body portion.
 17. Theinterconnect structure of claim 13, wherein the pores in the porousbottom portion has one first size, the pores in the body portion has onesecond size, and the one first size is smaller than the one second size.18. The interconnect structure of claim 13, wherein in the porous bottomportion, the pore size decreases to a positive value in two or moresteps in a depth direction of the porous low-k layer and is smaller thana pore size in the body portion.
 19. The interconnect structure of claim13, wherein the porous low-k layer further comprises a top portion ofthe same atomic composition that is located on the body portion and hasa density higher than the density of the body portion, and the topportion, the porous bottom portion and the body portion together havethe damascene opening therein over the first conductive layer.
 20. Theinterconnect structure of claim 19, wherein the top portion has no poretherein.
 21. The interconnect structure of claim 20, wherein a thicknessof the top portion does not exceed 250 Å.
 22. The interconnect structureof claim 19, wherein in the top portion of the porous low-k layer, thepore number density increases from zero or a positive value in two ormore steps in a depth direction of the porous low-k layer, and a poresize in the top portion is equal to that in the body portion.
 23. Theinterconnect structure of claim 22, wherein a fraction of the topportion where the pore number density is zero has a thickness no morethan 250 Å.
 24. The interconnect structure of claim 19, wherein thepores in the top portion has one first size, the pores in the bodyportion has one second size, and the one first size is smaller than theone second size.
 25. The interconnect structure of claim 19, wherein inthe top portion, the pore size increases from a positive value in two ormore steps in a depth direction of the porous low-k layer but is smallerthan a pore size in the body portion.